In a long-distance optical transmission system, large-capacity information transmission is economically realized by applying the wavelength division multiplexing (WDM) transmission technology, in which optical signals with a plurality of wavelengths are transmitted white being multiplexed in one optical fiber. Regarding a WDM transmission device, an increase in transmission rate per wavelength is now being studied for reducing the device cost. At present, a transmission rate of 10 Gbit/s per wavelength is available for practical use, and further, transmission at 40 Gbit/s is now being studied.
A major challenge in increasing the transmission rate from 10 Gbit/s to 40 Gbit/s involves improving tolerance to optical noise (signal/noise (S/N) ratio). In long-distance transmission, a transmission distance is limited by optical noise that is generated in a transmission path and an optical amplifier used in an optical transceiver. Under the same modulation/demodulation scheme for use, tolerance to noise at the transmission rate of 40 Gbit/s is as low as one-fourth of that at the transmission rate of 10 Gbit/s. For that reason, a modulation/demodulation scheme with enhanced tolerance to optical noise is now being studied for the transmission at the transmission rate of 40 Gbit/s. Such a scheme is typified by a configuration in which RZ-DPSK modulation/demodulation is employed and a balanced receiver utilizing a delay interferometer is used on a receiver side.
FIG. 10 illustrates an example of a light receiving circuit for demodulating RZ-DPSK signals. In FIG. 10, a 1-bit delay interferometer 102 includes a 1-bit delay element in one of a pair of optical waveguides, and outputs a pair of two optical signals 103 and 103 corresponding to a phase difference between adjacent bits. Two photodiodes (PDs) 104 and 104 convert the two optical signals 103 and 103 output from the interferometer 102 into intensity-modulated signals, respectively. An anode and a cathode of the two PDs 104 and 104 are connected to each other so that a difference between the two signals may be output to thereby demodulate the RZ-DPSK signals. By means of a transimpedance amplifier 100-1 having negative feedback (feedback resistor 106), the demodulated output signal is converted from a current signal into a voltage signal, and then amplified.
Further, FIG. 11 illustrates another configuration for demodulating RZ-DPSK signals. In FIG. 11, two PDs 104 and 104 are directly connected to a differential amplifier included in a transimpedance amplifier 100-2, and the differential transimpedance amplifier 100-2 outputs differences between the signals to thereby demodulate the RZ-DPSK signals.
Patent Documents 1 to 3 are mentioned as Prior Patent Documents relating to an optical signal receiving circuit. Each of Patent Document 1 (Japanese Unexamined Patent Application Publication (JP-A) No. 2006-50146) and Patent Document 2 (Japanese Unexamined Patent Application Publication (JP-A) No. 2007-158600) discloses a light receiving circuit for an RZ-DPSK signal. Although having different configurations, those light receiving circuits are basically equivalent to the circuit illustrated in FIG. 10. Patent Document 3 (Japanese Unexamined Patent Application Publication (JP-A) No. 8-331064) discloses a light receiving circuit that adjusts an offset so that signals may cross each other at the middle point of the amplitude. The method of Patent Document 3 completely differs from a method of adjusting levels of negative feedback closed loops according to this invention. As described above, the technology of this invention is not disclosed in each of the light receiving circuits disclosed in Prior Patent Documents, and therefore this invention cannot easily be presumed from the above Prior Patent Documents.